This invention relates to composite friction materials that exhibit high mechanical fatigue resistance and a stable coefficient of friction over a wide temperature range.
Various materials containing elastomers have been proposed for use as friction materials, for the purpose of increasing the resiliency of the composite material. Such property would be particularly desirable for use in heavy duty clutch and transmission applications, which are especially subject to high wear, deterioration and fatigue upon repeated engagement between the friction material and the mating member. Wear and fatigue conditions are particularly exaggerated when the friction material cannot accommodate high spots on the mating member.
It would also be desirable to provide a friction material having excellent characteristics that could be produced and formed into the desired shape without the use of sophisticated or expensive machinery and processing methods. Many friction compositions are stiff or inflexible and require precision molding or other special production techniques.
Heretofore, the foregoing criteria have not been adequately met by a single type of friction material. Generally, materials that are easily fabricated exhibit poor performance, especially in heavy duty applications. Other materials considered suitable for heavy duty friction requirements are difficult or costly to produce or fabricate.
As examples of the foregoing, in order to obtain a stable coefficient of friction over a wide temperature range, industry has resorted to the use of inorganic friction materials such as sintered bronze. The production of these materials, however, requires the use of heavy presses and other costly equipment and processing common to powder metallurgy. Also, the resulting material has a high modulus and tends to fatigue and erode under heavy loads, due in many cases to the inability to conform to the mating member.
Various friction compositions containing rubber and a phenol-formaldehyde resin have been proposed, as illustrated in U.S. Pat. Nos. 2,149,483, 2,599,826, 2,686,140, 2,910,449, 3,152,099, 3,344,094, 3,365,041 and 3,673,276. Generally, these materials have been found to deteriorate under high peak and bulk temperature conditions created by the sustained or heavy loading forces experienced while under operating conditions. As a result of this decomposition, the physical properties of the material undergo undesirable change and require replacement.
A recent improvement described in U.S. Pat. No. 3,898,361 is the use of a fluroelastomer as a binder in a friction material. Although the resultant material exhibits very good physical properties, the method of manufacture requires costly high pressure precision molding, and the basic material utilized is expensive.